Steerable bottom hole assemblies include a positive displacement motor (PDM) or “mud motor” which has a bend angle that allows a drilling operator to build at a desired rate when “sliding” the non-rotating motor housing. The bend angle of the bent sub or bent motor housing, in combination with the contact points of the bottom hole assembly (BHA) with the well bore wall, thus largely determines the build rate for drilling the curved borehole section. When the drill string and the motor housing are rotated, the drilling operator drills a straight or tangent section of the borehole. The rate of penetration (ROP) of the bit drilling through the formation is frequently significantly less when “sliding” to build rather than rotating the bottom hole assembly from the surface to drill the straight borehole sections. Since ROP translates to time and money, drilling operators prefer to maximize the time when the BHA is rotated for drilling straight, and minimize the time of building. For various reasons, high build rates are generally preferred, but the bend angle is preferably low to minimize problems when the BHA is rotated. A steerable bottom hole assembly is disclosed in U.S. Pat. No. 6,269,892. This patent discloses the benefits of a steerable bottom hole assembly with a long gauge bit, i.e., a bit having a gauge section with an axial length of at least 75% of the bit diameter.
Underreaming tools within the bottom hole assembly are used to enlarge a section of the well bore below a restriction. Advantages of underreaming are disclosed in an article “Simultaneous Drilling and Reaming Saves Rig Time”, Oil & Gas Journal, Nov. 27, 1989. Conventional underreaming tools use three or more cutting arms that are moved outward in response to fluid pressure within the tool. A reamer designed for hole opening while drilling is the NBR reamer offered by Security DBS, a Halliburton Company.
When reamers are used in a conventional steerable bottom hole assembly immediately above the bit and below the motor housing, the bottom hole assembly has very poor steerability. More particularly, it is difficult to accurately predict the build rate when sliding, regardless of the positioning of stabilizers above the reamer. Because of high vibration, the azimuth of the curved borehole formed while sliding may be inaccurate. Moreover, borehole quality is generally poor due to high vibration, which prohibits the motor housing from “holding steady” in the well.
Because of the disadvantages of a steerable bottom hole assembly and reamer combination discussed above, operators have used bi-center bits rather than reamers for performing the underreaming operation. Bi-center bits are discussed in “Application of Bi-Center Bits in Well-Deepening Operations, IADC/SPE 19921, page 131. While bi-center bits may be a better solution than a reamer in some applications, significant problems remain when using a bi-center bit. Vibration due to the bi-center bit is significant, and the bottom hole assembly is also difficult to steer. Vibrations are particularly high when drilling relatively hard formations. High vibration not only increases the likelihood of failure in the bottom hole assembly, but is generally indicative of poor borehole quality. High vibration typically results in excessive bit wear. Drilling operators are constantly seeking techniques which will result in better hole quality when drilling with a steerable downhole motor. The bottom hole assembly also should have good steerability and preferably be able to drill at a relatively high ROP.
The disadvantages of the prior art are overcome by the present invention, and an improved steerable bottom hole assembly and method are hereafter disclosed for underreaming a section of borehole with relatively low vibration and high borehole quality.